Polyolefin polymers are in general hydrophobic. Polarity of an ethylene based polymer can be obtained by copolymerization of ethylene and a polar monomer such as vinyl acetate, alkyl acrylate, (meth)acrylic acid, etc. via a free radical polymerization process. Ethylene copolymers such as ethylene vinyl acetate and ethylene alkyl acrylate copolymers are widely used in industry due to their diverse properties, low cost, and broad availability. As the polar comonomer content increases, desirable properties such as adhesion, flexibility, and optical clarity improve. However, there are trade-offs. As the vinyl acetate content increases in copolymers of ethylene and vinyl acetate both the melting temperature and the crystalline content drop accordingly. This imposes a restriction on the usage for applications where physical integrity is needed up to a specific temperature, such as 80° C. Also an ethylene copolymer with a low melting point and low crystallinity as indicated from the heat of fusion is difficult to manufacture due to its stickiness and softness, resulting in blocking. Furthermore, an ethylene copolymer even containing a large amount of polar monomers may still be deficient in some key performance that limits its usage.
Thus it is desirable to obtain more polar polymeric compositions that overcome the thermal deficiencies of ethylene copolymers with high polar comonomer content and provide improved properties that arise from their higher polarity. For example, ethylene/vinyl acetate and ethylene/alkyl acrylate copolymers have to be modified to provide HF activated welding.
High frequency (HF) or radio frequency (RF) welding is useful in flexible packaging, flexible bag production, textile lamination, and in producing automotive components such as headliners and sunvisors. HF welding is an alternative to heat-bonding methods for adhering a film to a substrate such as the film itself, another film, or a textile fabric. HF welding involves heating only a HF-active component or HF-active layer of a structure such as a multilayer film sufficiently to soften that component. The selective heating is accomplished by treatment with high frequency radiation. In contrast, heat-bonding methods require transferring heat through an entire structure to soften a bonding layer or component in the structure. In each case, the softened layer or component subsequently bonds the film structure to a substrate.
HF welding can be advantageous over heat-bonding methods. First, HF welding can bond a film in a fraction of the time required for heat-bonding methods. Second, HF welding is less likely to degrade thermally sensitive materials, such as oriented films and thermally sensitive dyes. Third, bonding complex shapes is possible using HF welding.
Flexible polyvinyl chloride (f-PVC) has been used in HF-active films due to its HF sealing capability, vapor and gas barrier properties, and flexibility. Films of f-PVC typically include a plasticizer, typically a phthalate plasticizer, to enhance film flexibility. The plasticizer can migrate out of the polymer over time, decreasing film flexibility and potentially contaminating materials in contact with the film. A desire for long-term flexibility and concern about the environmental impact of halogenated polymers, such as f-PVC, make it desirable to have other HF-active polymers as an alternative to f-PVC.
Olefin/acrylate copolymers and olefin/vinyl ester copolymers demonstrate some HF activity when they contain greater than about 12 weight percent of a polar comonomer such as alkyl acrylate or vinyl ester. Such a high level of polar comonomer reduces a polymer's crystalline melting point (Tm) below 100° C., and generally below 90° C. Polymers having such a low Tm are not suitable for use in many articles where a film must maintain physical integrity through multiple exposures to temperatures around, and particularly above, 100° C. Examples of such articles include many textiles, such as clothing articles, which are subject to repeated washing and drying cycles. Additional examples of such articles include sun visors for automotive interiors. Olefin/acrylate and olefin/vinyl ester copolymers also tend to have a lower dielectric loss factor (DLF) than f-PVC. A lower DLF means more HF energy is necessary to weld the copolymers with HF than needed for f-PVC. Addition of HF-active fillers can help increase the DLF of a copolymer film, but may do so at the expense of physical properties such as tensile strength.
Commercially cost effective HF-active film-forming polymer compositions and HF-active films that have long-term flexibility and Tm greater than 90° C. are desirable as alternatives to f-PVC compositions and films. Preferably, the polymer compositions and films are also essentially halogen-free.
Patent Application Publications WO2002/102898 and US2003/0021945 disclose a blend of 20 to 80 percent, by weight, of low weight-average molecular weight copolyester with a carboxyl-containing polyolefin that has a dielectric loss (DLF) factor of 0.05 or more at 27 MHz and 23° C. This composition is reported to be HF-active, but the blend has a low melting point, limiting temperature resistance. Also, blends of EVA and low molecular weight polyester are immiscible and result in poor optical clarity.
U.S. Patent Application Publication 2005/0255328 discloses potassium-neutralized ionomer compositions modified with fatty acids and polyols or polyesters that have RF weldability.
U.S. Pat. No. 7,935,765 describes miscible blends containing an ethylene copolymer comprising ethylene and maleic anhydride, maleic acid monoesters, maleic acid diesters, fumaric acid monoesters, or mixtures thereof and an ethylene copolymer comprising ethylene and vinyl acetate, alkyl acrylates, alkyl methacrylates, carbon monoxide and mixtures of two or more thereof. U.S. Pat. No. 7,879,949 discloses similar blends that are reported to be HF-weldable and have higher temperature resistance compared to high polarity ethylene copolymers due to their higher melting points. However, the polarity of the blends is still not strong enough to attain robust HF welding and certain desirable properties, such as antistatic, etc.
It is desirable to find additional HF-weldable compositions that can remain flexible without the inclusion of a plasticizer and exhibit improved properties over known HF-weldable compositions.
Antistatic property is also important for many applications. Generally, a fabricated article made from a polymeric material can become statically charged, and the surface can attract and hold charged particles such as dust in the air. In some cases an article can become damaged and/or otherwise devalued by the adhesion of electrostatically charged species.
Except for polymers of inherently high polarity, most organic polymeric materials lack adequate antistatic resistance especially at low humidity. This is especially true for polyolefin materials, such as polypropylene, polyethylene, and ethylene copolymers, etc. Many attempts have been made to address this issue from aspects of performance, cost, and ease of conversion to final products. A common approach to enhancing the antistatic properties of thermoplastics is to introduce low-molecular weight antistatic agents into the polymeric material by compounding prior to or during the manufacturing of articles, e.g., by means of molding or film-forming processes. Antistatic agents work by migrating to the external polymer surface of the manufactured articles because of their high volatility and poor compatibility with polymer composition. They form a continuous film on the surface of the polymers. Therefore, the incorporation of a low-molecular weight antistatic agent for achieving antistatic is not trouble free. Materials that come in contact with the composition can become contaminated due to bleeding of the antistatic agent out of the composition and/or the antistatic agent effect can be deteriorated with time.
Another commonly practiced approach is to add permanent, non-diffusing antistatic agents based on polymers with high antistatic properties. For example, block copolymers based on polyether-block-amide (commercially supplied by Atochem under the PEBAX® tradename) may be used as permanent antistatic agent by compounding with an isolative polymer to lower the surface resistivity.
While polyether-block-amides may perform well in certain polymer systems, they do not perform well with polyolefin systems. Due to their poor compatibility with polyolefin materials, a third polymer serving as a compatibilizer may have to be added to insure the antistatic function of the polyolefin matrix. This may confine the composition to be processed in a narrow processing window for attaining the antistatic performance. Polyetheramide block copolymers do not have suitable direct adhesion to other substrates, especially polyolefin substrates. Their high cost is another issue for their use as permanent antistatic agents.
Potassium-neutralized ionomer compositions have been developed to act as permanent antistatic agent in blending with polyolefins (see e.g. PCT Patent Application publication WO2004-050362). K-ionomer compositions may function well as permanent antistatic agents, but they are difficult to produce and handle in initial manufacture and when converting into final products.
Overall, a satisfactory solution for polyolefin-based materials with adequate antistatic performance cannot be achieved without some deficiencies.